Radosław P. Wesołowski

Pillared graphene as a gas separation membrane

Graphene and carbon nanotubes are considered as future materials in various fields, including adsorption, accumulation and separation processes, and so are hybrid materials combining their properties. This paper reports our study on separative abilities of 3-D network structures consisting of graphene planes pillared with nanotube fragments. Results of molecular dynamics simulations confirm that such materials can be successfully applied as membranes in relation to noble gas mixtures. A simple explanation of the mechanism underlying the process is proposed.

Phenol adsorption on closed carbon nanotubes.

We present the results of systematic studies of phenol adsorption on closed commercially available, unmodified carbon nanotubes. Phenol adsorption is determined by the value of tube-specific surface area, the presence of small amount of surface groups influence adsorption only in very small amount. Phenol can be applied as a probe molecule for comparative analysis of tube surface areas. Tube curvature influences adsorption from solution, i.e., we observe increasing adsorption energy (and slower desorption process) with the decrease in tube curvature. This is in full accordance with molecular simulation results.

Ar, CCl4 and C6H6 adsorption outside and inside of the bundles of multi-walled carbon nanotubes—simulation study

This is the first paper reporting the results of systematic study of the adsorption of Ar, C(6)H(6) and CCl(4) on the bundles of closed and opened multi-walled carbon nanotubes. Using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, we also study the effect of the introducing defects in the external and internal walls of osculating and separated nanotubes on Ar diffusion and on adsorption of all three adsorbates. The Ar diffusion coefficients obtained are very sensitive to the presence of defects. Simulated isotherms are discussed to show the relation between the shapes of the high resolution alpha(s)-plots and the mechanisms of adsorption. From obtained data, as well as from geometric considerations, from the VEGA ZZ package, and from simulations (ASA), the values of surface areas of all nanotubes are calculated and compared with those obtained using the most popular adsorption methods (BET, alpha(s) and the A,B,C-points). We show that the adsorption value for the C-point of the isotherm should be taken for the calculation of the specific surface area of carbon nanotubes to obtain a value which approaches the absolute geometric surface area. A fully packed monolayer is not created at the A-, B- or C-points of the isotherm; however, the number of molecules adsorbed at the latter point is closest to the number of molecules in the monolayer as calculated via the ASA method, the VEGA ZZ package or from geometric considerations.

Molecular dynamics of zigzag single walled carbon nanotube immersion in water

The results of enthalpy of immersion in water for finite single-walled carbon nanotubes are reported. Using molecular dynamics simulation, we discuss the relation between the value of this enthalpy and tube diameters showing that the obtained plot can be divided into three regions. The structure of water inside tubes in all three regions is discussed and it is shown that the existence of the strong maximum of enthalpy observed for tube diameter ca. 1.17 nm is due to freezing of water under confinement. The calculations of hydrogen bond statistics and water density profiles inside tubes are additionally reported to confirm the obtained results. Next, we show the results of calculation for the same tubes but containing surface carbonyl oxygen groups at pore entrances. A remarkable rise in the value of enthalpy of immersion in comparison to the initial tubes is observed. We also discuss the influence of charge distribution between oxygen and carbon atom forming surface carbonyls on the structure of confined water. It is concluded for the first time that the presence of surface oxygen atoms at the pore entrances remarkably influences the structure and stability of ice created inside nanotubes, and surface carbonyls appear to be chaotropic (i.e. structure breaking) for confined water. This effect is explained by the pore blocking leading to a decrease (compared to initial structure) in the number of confined water molecules after introduction of surface oxygen groups at pore entrances.

Water nanodroplet on a graphene surface-a new old system.

The major subject of our study is the accuracy of contact angle calculations. Reporting new simulation data for graphene-water systems, we show that the majority of previously reported data should be treated with caution, since the proper contact angle can be recorded only after a sufficiently long simulation time. It has been proven that-if one wants to gain accuracy greater than 0.1°-long calculations (exceeding 50 ns) are required. Finally, we also show, using both a Groningen Machine for Chemical Simulations (GROMACS) package and our new molecular dynamics (MD) code, that the changes in the contact angle, caused by graphene bottom layer rotation, are within the range of calculation error. We also propose a novel definition of the bottom of the droplet as the height where the density is half the density of liquid water. This new definition is applied in the method of the contact angle calculation from the MD simulation data.